Functional hypothalamic amenorrhea (HA) is a reversible form of hypgonadotropic hypogonadism that is defined by 3-6 months of amenorrhea in the absence of pregnancy, androgen excess, hyperprolactinemia or thyroid or other endocrine dysfunction. Epidemiologic studies that define HA as 3 months of amenorrhea in the absence of a history of oligoamenorrhea (which is more consistent with polycystic ovarian syndrome), suggest a population prevalence of 4.5%. HA is manifest by variable patterns of deficient pulsatile LH secretion, indicative of GnRH secretory dysfunction. The clinical course of HA may change over time with withdrawal bleeding in response to a progestin or follicle development in response to clomiphene citrate (both indicative of some degree of estrogen exposure) at some times over the course of the disorder, but not at others. The pattern of pulsatile LH secretion may also change over time. Studies in select populations indicate that the prevalence of HA is significantly higher in association with exercise, subclinical eating disorders and younger reproductive age. Other studies suggest that psychological characteristics, stress and/or activation of the hypothalamic-pituitary-adrenal (HPA) axis may also play a role in HA. Peripheral signals convey information about feeding and overall nutritional state to the hypothalamus that influence not only satiety and metabolic balance, but also reproductive control. Similarly, both nutritional and psychological stress impact reproductive pathways and there is evidence that at least some metabolic signaling operates through stress pathways. It is of both clinical and scientific importance that there is significant clinical heterogeneity in the response of individual women to apparently similar risk factors. For example, in the Frisch studies of weight for height, in a girl of 165 cm with secondary amenorrhea, the 95% confidence limits associated with resumption of menses ranged from 43-60 kg. Likewise, in studies of middle distance runners, percent amenorrhea was positively associated with miles run per week, but even at 80 miles per week, only 50% of athletes were amenorrheic. Inter-individual variability in the response to mild stress in the setting of metabolic deficiency was also noted in the well-controlled non-human primate model of HA 52. Thus there is significant evidence that women vary in the susceptibility of the reproductive axis to exercise, weight changes, and stress. A relatively small group of patients with HA were sequenced for 7 GnRH-related genes to determine whether mutations in these genes might extend from complete patients with complete GnRH deficiency to milder phenotypes. This study identified six heterozygous mutations in 7 of the 55 HA women for an overall prevalence of 13%. These variants were not identified in 422 healthy control women. This study indicated that heterozygous rare variants in genes associated with congenital forms of HH may also be seen in patients with secondary amenorrhea and functional HA. More recently rare sequence variants (RSVs) in genes identified in KS/nIHH were shown to be overrepresented in patients with constitutional delay of puberty (CDP) when compared with the publicly available databases, providing another setting in which these genes identified in the rare disorders of KS/nIHH may contribute to more common disorders. These published studies support our overall hypothesis that genetic susceptibility may contribute to the variability in the reproductive system response to physiologic stresses that results in HA. However, further studies are needed due to both the small sample size and the analysis strategy used in the initial study of HA and GnRH genes that would not be acceptable by todays standards. The above study involving subjects with CDP provides some confidence that our previous findings in patients with HA may in fact be confirmed with more current analytic methodologies. Such a finding could have implications for screening women with amenorrhea with or without risk factors for HA and may allow for improved prediction of fertility outcomes for women with HA.